The invention relates to the storage and retrieval of digital video data on a video tape recorder (VTR).
A system disclosed in British Patent application 7,930,222 (Publication 2,059,713) and U.S. patent application Ser. No. 172,721 relates to a video information system which allows video digital data to be recorded onto a standard analogue video tape recorder.
With modern types of VTR, there is a tendency for degradation due to signal drop out (resulting from imperfections on the tape for example). When such tapes are used for duplication purposes, any errors are compounded.
FIG. 1 shows a typical defect on the video tape 10. The scratch 11 extends across the path of several recorded fields and the analogue information from the tape on playback is visibly degraded as a result of the drop outs 12 due to this scratch. Several drop outs on a line will give the result of a horizontal stripe on the picture. Thus because such a drop out has no vertical structure, using the basic recording technique described in the aforementioned patent application will mean that the serially recorded data will result in errors occuring whenever a drop out occurs. It is known to compensate for drop outs on normal video information by using data from an adjacent line above or below the drop out to replace any data lost (see U.K. Pat. No. 1,436,757 and U.S. Pat. No. 3,949,416 for example). Whilst this works well for normal analogue picture information, in the above information system using digital data its configuration is such that adjacent data will not correspond to the data missing in the drop out so this type of error correction is ineffective.